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I am reading Asimov's Foundation series again and on multiple occasions the Empire's decay is mentioned and that it is starting from the most distant systems (in the Outer Worlds).

Looking in current today's world, distance is paramount to maintain control because it takes quite some time to deploy troops and equipment. However Foundation's world has the jump drive (faster-than-light travelling) so travelling very long distances should not be an issue or am I missing something?

Theoretically, distance should not matter unless jumps are somehow limited distance-wise.

Question: Why do distances seem to matter in the Foundation world?

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  • As an aside, every author who tries to do stories that parallel historical events has to worry about the effects of travel and communication delays. In science fiction it is conventional to build in an explanation for why the delays take the form they do. And generally early in the story, at that. If you are paying attention you can use this data to anticipate possible turning points int he plot.
    – dmckee --- ex-moderator kitten
    Apr 27, 2019 at 19:03

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Jumps indeed are limited in distance. Near Trantor, they are extremely limited.

The stars were as thick as weeds in an unkempt field, and for the first time, Lathan Devers found the figures to the right of the decimal point of prime importance in calculating the cuts through the hyper-regions. There was a claustrophobic sensation about the necessity for leaps of not more than a light-year.
— Foundation and Empire

The first part of any journey out from Trantor to the outer star systems, and the last part of any return journey, involves passing the Galactic Core region, where hyperspace travel is slow. It's slow because it involves lots of short jumps, and because, until the invention of the Lens as described in Second Foundation, each of those jumps involves a lot of lengthy work.

It was because of that, that the Lens had performed a near-revolution in interstellar travel. In the younger days of interstellar travel, the calculation of each Jump through hyperspace meant any amount of work from a day to a week — and the larger portion of such work was the more or less precise calculation of "Ship's Position" on the Galactic scale of reference.
— Second Foundation

Advances like this happen over the course of the series. You will for example find, as a plot point, an advance in hyperspace jumping that enabled one military force to defeat another in battle. The changes in technology as the Empire falls and through the Interregnum, and an Empire where the capital world is physically remote from and difficult to reach from the periphery, are some of the plot drivers in Asimov's famous "human-only Galactic Empire" setting.

So the answer is: because constructing the setting that way drives the plot.

Further reading

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When describing Trevize's ship capabilities and performance in "Foundation's Edge", he is awestruck at the way the ship can chain jumps with computation time close to zero and deviation from target coordinates also negligible, thus pointing to should-be normal, expected behaviour of a ship in-universe.

  • End point of the Jump needs to be accurately calculated in advance. Upon emergence in normal space, actual end point of emergence needs to be validated in the assumption a significant deviation from expected point has happened. This is easy but not instantaneous.
  • The longer the Jump, the bigger the deviation. I don't remember energy constraints been part of a Jump concerns, but implication is strong if you try to single Jump from Trantor to Terminus you might well end up half-way to Andromeda (exaggerated for laughs).
  • The actual point in space where the ship is after the Jump needs to be used as input to re-calculate again the next jump in the series. I kind of remember this was expected to be easy, but in the hours range.

So in short, a hyperspace trip is a pre-calculated set of jumps which tries to reach a perfect compromise between distance in each jump and minimizing emergence errors, but after each of the jumps the remaining jumps need to be recalculated again with actual coordinates.

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    It is mentioned several times that trips can take weeks if not months to complete.
    – JRE
    Apr 25, 2019 at 8:24
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    History repeats itself: the Empire in this regard resembles the Roman Empire in that troops can take weeks or months to traverse it, hence it's non-trivial to secure the borders.
    – Hans Olo
    Apr 25, 2019 at 8:44
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    @Rebel-Scum that resemblance was not an accident.
    – Daniel Roseman
    Apr 25, 2019 at 10:33
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    @DanielRoseman Indeed, if I remember well Asimov stated at some moment one of the main sources of inspiration for the original trilogy was Gibbon's "The History of the Decline and Fall of the Roman Empire"
    – Seretba
    Apr 25, 2019 at 11:52
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    If I recall it correctly, gravity wells makes jumps more complicated, so it is necessary to travel for several hours/days to make distance from planets and stars before the first jump.
    – Pablo Lozano
    Apr 25, 2019 at 13:27
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I'd like to add to the excellent answers by @Seretba and @JdeBP that you have to consider not only the time that it takes to reach the Empire's Edge but also the area that the Empire has to cover to maintain control over the territories.

From a mathematical point of view, if you model the Empire's territories as a circle with Trantor as its center, you can see that increasing the distance to the farthest point (the radius' length) dramatically increases the area that you need to maintain control over.

So, it makes sense for an Empire in decay to shrink its size to better control a smaller territory with the same resources. This is compounded by the fact that travel is not instantaneous, which means that the assets near Trantor are more valuable than those far away, so more resources are poured into controlling them.

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  • Consider the volume rather than the area. Cubed instead of squared. Much worse.
    – JRE
    Apr 25, 2019 at 19:26
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    @JRE, for all practical purposes, the Milky Way is a two-dimensional object, not a three-dimensional one. There's not much above or below the galactic plane that's worth going to.
    – Mark
    Apr 25, 2019 at 23:17
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    @JRE Actually, not worse. A spherical sphere of influence is better, because you can fit more worlds within the same range. A flat disk would have the same distances, but fewer worlds. And as Mark noted, the Milky Way, while not exactly flat, is rather thin - 2 000 ly isn't exactly a negligible width, but compared to the diameter of 200 000 ly...
    – Luaan
    Apr 26, 2019 at 6:21

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